Conventional liquid crystal devices, as exemplified by twisted nematic or supertwist nematic devices, utilize a quadratic electrooptic effect, i.e. an effect sensitive to the magnitude but not to the sign of an applied electric field. The response of the liquid crystal material to the external field is, in this case, of dielectric nature, and the decisive material parameter is the dielectric anisotropy, that is, the difference between the value of the dielectric constant along the long molecular axis and the value perpendicular to that axis, which is also the optic axis. According to whether this anisotropy is positive or negative, an applied electric field will have the tendency to align the material in such a way that the optic axis is along or perpendicular to the field, respectively. According to its nature, this response increases proportional to E.sup.2, the square of the applied field.
Since about a decade a radically different liquid crystal technology is growing based on ferroelectric liquid crystals, first described by R. B. Meyer et al. in Journal de Physique, volume 36, pages L69 to L71, 1975. The first ferroelectric liquid crystal patent described the so-called surface-stabilized ferroelectric liquid crystal (SSFLC) and was filed by N. A. Clark and S. T. Lagerwall in 1980 and issued in 1983 as U.S. Pat. No. 4,367,924. The corresponding first announcement of high speed liquid crystal devices was made by the same authors in Applied Physics Letters, volume 36, pages 899 to 901, 1980. In these devices which are characterized by microsecond or submicrosecond speed, and by the completely new feature of symmetric bistability, the active electro-optic effect is a linear one, i.e. sensitive to the sign of the field. Whereas in a twisted nematic device the two distinct optical states are characterized by the field being ON or OFF, the SSFLC device can be driven between its two distinct states by changing the sign of the applied electric field.
Later linearly responding devices without bistability were described by Lagerwall et al. in U.S. Pat. No. 4,838,663, filed in 1987. The devices use different liquid crystal phases, being orthogonal smectics rather than tilted smectics used in the ferroelectric devices. These paraelectric phases, similar to the solid crystal case, typically exhibit a so-called soft mode, which is detectable as a pretransitional effect immediately before entering the ferroelectric phase. In the liquid crystal case such an effect was described first by S. Garoff and R. B. Meyer in Physical Review Letters, volume 38, page 848, from 1977 and they coined the word electroclinic for the response. This response means that the molecular axis n, which is also the optic axis, rotates a certain angle .theta. when an electric field is applied perpendicular to n, cf. FIG. 1. When the field direction is inversed, the induced tilt angle is in the opposite sense. The induced tilt .theta. is proportional to E, but in the case of Garoff et al., so small that it requires phase sensitive methods for even being detected. In U.S. Pat. No. 4,838,663, which uses a different geometric configuration and explores a different temperature regime, the induced tilt is orders of magnitude larger and the effect is also distinguished by the fact that the response time is independent of the applied field, which makes the field an excellent control variable for .theta. and thereby for a grey scale in devices up to very high frequencies.
Recently it has been discovered that the electroclinic effect can be detected not only in the orthogonal smectic phase, but also in the higher-laying nematic phase, that is in the very opposite regime to that investigated by Garoff et al. This was first reported by one of the authors (Komitov) at the International Conference on Optics and Interfaces in Liquid Crystals, held in Torino, Oct. 14-20, 1988; cf. FIG. 2. Independently, the same finding was reported by Z. Li, R. Petschek and C. Rosenblatt in the Physical Review Letters, volume 62, pages 796 to 799, 1989. Although the induced tilt is larger in both these cases as compared with the Garoff et al. case, it is still by far too small to be useful in any practical device, being of the order of 10.sup.-2 degrees. In order to apply the electroclinic effect to any nematic device, the response has to be amplified by at least two orders of magnitude. In recognizing that, for its utilizability, the phenomenon depends on surface control, we have been able to find means to make it useful for device applications.